Structure for relieving stresses on coaxial cable seals



United States Patent 3,172,944 STRUCTURE FOR RELIEVING STRESSES ON COAXIAL CABLE SEALS Roger P. Hynes, Murray Hill, and Edward .I. Walsh, Morris Plains, N.J., assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed July 2, 1963, Ser. No. 292,243 3 Claims. (Cl. 174-28) This invention relates to hermetic seals for coaxial cables.

The application of N. C. Wittwer, Jr., Serial No. 214,- 302, filed August 2, 1962, describes the use of a coaxial cable as an electron collector. A modulated electron beam is projected through a slot in the outer conductor of the coaxial cable to be collected by the inner conductor. As the electrons travel between the outer and inner conductors, the beam modulations excite an electromagnetic wave that is transmitted by the cable to an appropriate load. The part of the cable that is used to collect the electrons must be maintained in a near vacuum by means of hermetic seals in order to prevent ion interference with the electrons.

Hermetic coaxial cable seals made of glass have been used in the past for various purposes. Such seals do not ordinarily interfere with a propagating wave if the thickness of the seal is much smaller than the wavelength of the propagating wave. However, such seals have been found to be generally unsatisfactory for use in the Wittwer device when it is being used to detect and transmit waves at frequencies above one kilomegacycle per second because a suitably rugged glass seal cannot be made thin enough to avoid attenuation and distortion of waves hav ing such short wavelengths.

It is therefore an object of this invention to reduce the attenuation and distortion of high frequency electromagnetic waves by hermetic coaxial cable seals.

These and other objects of the invention are attained through the use of a thin ceramic water as a hermetic seal within a coaxial cable. The wafer has a central aperture through which the inner conductor of the coaxial cable extends. The outer periphery of the wafer is sealed to the outer conductor and the periphery of the aperture is sealed to the inner conductor. A ceramic wafer which is less than .04 inch thick is substantially transparent to electromagnetic waves in the lower kilomegacycle frequency range; as the frequency requirements increase, thinner seals are normally required to minimize attenuation. Because of the inherent mechanical strength of ceramic, a wafer as thin as 0.01 inch can withstand atmospheric pressures and maintain a vacuum within a section of coaxial cable. It has been found, however, that thermal stresses between the ceramic wafer and the metallic coaxial cable may crack the wafer during the sealing process.

In accordance with a feature of one embodiment of the invention, that portion of the coaxial cable which contains the hermetic seal is constructed of ceramic. The inner surface of the outer conductor portion, and the outer surface of the inner conductor portion, are metallized with a thin layer of conductive metal in order to preserve the transmission characteristics of the coaxial cable. For example, a plating as thin as .003 inch is sufiiciently thick to transmit all electromagnetic waves in the microwave frequency range. Thermal stresses in the wafer during the sealing process are avoided because the coaxial cable portion is made out of the same material as the wafer and because the metal plating is too thin to exert substantial stresses on the wafer.

Analysis has shown that most of the thermal stresses between a ceramic wafer and a conventional coaxial cable are concentrated at the junction between the wafer and the inner conductor. It is a feature of another embodiment of this invention that the stresses at this junction be relieved by employing an inner conductor that is hollow in the region of the junction. By making the wall of the hollow inner conductor thinner than the wafer, the comparatively flexible metallic inner conductor wall will absorb most of the thermal stresses.

These and other objects and features of our invention will be more clearly understood from a consideration of the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a sectional view of one embodiment of the invention; and

FIG. 2 is a sectional view of another embodiment of the invention.

Turning now to FIG. 1, there is shown a ceramic coaxial cable portion 10 comprising a hollow outer cylinder 11 surrounding an inner cylinder 12. The purpose of ceramic cable portion 10 is to support a ceramic Wafer 14 which acts as a hermetic seal. The ceramic cable portion is sealed to one end of a coaxial cable 15 comprising an outer conductor 16 and an inner conductor 17 which are most commonly made of copper. The coaxial cable is adapted to transmit electromagnetic waves at very high frequencies, as for example, from one to thirty kilomegacycles per second. A projection 19 on the ceramic inner cylinder fits into an opening in inner conductor 17, while the ceramic outer cylinder includes a projection 20 for receiving a cylindrical sealing band 21. Projection 20 of the ceramic portion is sealed to the coaxial cable 15, as by brazing, to provide a rugged hermetic junction between the cable and the ceramic cable portion. Typically, the other end of the ceramic portion 10 abuts against another coaxial cable section, but this junction does not necessarily require a hermetic seal.

In accordance with the invention, the inner surface of the outer cylinder 11 is plated with a layer 22 of conductive material, while the outer surface of inner cylinder 12 is plated with a similar layer 23. The plating may be made, for example, by spraying or painting onto the surfaces a known mixture of percent manganese and 20 percent molybdenum in a suspension vehicle of pyroxyline, amyl acetate and acetone, firing the elements in a suitable atmosphere, and then electroplating any suitable conductor such as nickel, silver, or copper to a thickness of approximately .003 inch. According to The Microwave Engineers Handbook, Horizon HouseMicrowave, Inc., 1963, the depth of penetration, or skin depth, of copper at a frequency of 10 kilomegacycles per second is less than .001 millimeter; it can therefore be appreciated that the layers 22 and 23 may be made much thinner if so desired without affecting the transmission characteristics of the coaxial cable section 10.

Wafer 14 may be hermetically sealed to coaxial cable section 10 by metallizing its outer periphery and aperture periphery, inserting rings of brazing material near the junctions of the wafer with the cable portion, and heating the assembly to melt the braze material. There will, of course, be no substantial difference of thermal expansion between the wafer 14 and the coaxial cable section 10 because they are both made of the same material. Moreover, the layers 22 and 23 will not exert substantial thermal stresses on the wafer because they are much thinner than the wafer. It has been found that ceramic wafers having a thickness of .030 inch, and as thin as .010 inch, can be hermetically sealed by this method; these wafers are substantially completely permeable to electromagnetic waves having frequencies in excess of one kilomegacycle per second and yet are rugged 3 enough to withstand normal stresses and atmospheric pressure. The inner and outer ceramic cylinders are thick enough to absorb any incidental thermal stresses without damage; for example, the hollow outer cylinder may have a wall thickness of .04 inch while the inner cylinder has a diameter of .136 inch.

The embodiment shown on FIG. 2 comprises a coaxial cable section 25 constructed to take advantage of the discovery that most of the thermal stresses exerted by a conventional coaxial cable on a thin ceramic wafer during sealing are concentrated at the junction between the wafer and the coaxial cable inner conductor. Cable section 25 comprises an outer conductor 26 and an inner conductor 27 both of which are made of copper or some other suitable conducting material. Part of the inner conductor 2'7 is hollowed out to form a thin wall 28 to which the thin ceramic Wafer 29 is hermetically sealed. In accordance with the invention, the wall 28 is thinner than the wafer 29. Under this condition, the relatively flexible wall 28 will buckle slightly during the sealing process and thereby absorb most of the thermal stresses at its junction with wafer 29. We have found that a Wall thickness of .008 inch is satisfactory in conjunction witha wafer having a thickness of .030 inch. Coaxial cable section 25 is adapted to abut against one end of a conventional coaxial cable in a manner similar to that described above.

Although our invention was conceived to solve a problem encountered in the construction of a coaxial cable electron collector, it is apparent that the invention could be employed in any apparatus in which a hermetically sealed coaxial cable section is desired. The explicit dimensions given are intended to be illustrative of preferred embodiments rather than being definitive of the invention. Further, various combinations of the disclosed features could be used in a single device. 7 For example, a metallized ceramic outer cylinder could be used in conjunction with a hollow metal inner conductor; a metal outer conductor could be used with a rnetallized ceramic inner conductor. Various other modifications may be made to the embodiments shown and described without departing from the spirit and scope of the invention.

What is claimed is:

1. In combination:

a coaxial cable having a metal outer cylinder surrounding a metal inner cylinder;

a coaxial cable section having a ceramic outer cylinder surrounding a ceramic inner cylinder;

said section being fitted hermetically to said coaxial cable;

a first conductive coating on the inner surface of the ceramic outer cylinder;

a second conductive coating on the outer surface of the ceramic inner cylinder;

and a ceramic wafer hermetically sealed to the inner and outer ceramic cylinders of the coaxial cable section.

2. In combination:

a coaxial cable having a hollow conductive outer cylinder surrounding a conductive inner cylinder;

said cable being adapted to transmit electromagnetic waves having frequencies exceeding one kilomegacycle per second;

a coaxial cable section having a ceramic outer cylinder surrounding a ceramic inner cylinder;

said section being abutted hermetically to one end of the coaxial cable;

a layer of conductive material bonded to the inner surface of the ceramic outer cylinder;

a layer of conductive material bonded to the outer surface of the ceramic inner cylinder;

said layers having a thickness of less than .010 inch;

and a ceramic wafer hermetically sealed to the inner and outer ceramic cylinders of the coaxial cable section;

the thickness of the wafer being less than .040 inch.

3. In combination:

a coaxial cable having a hollow conductive outer cylinder surrounding a conductive inner cylinder;

said cable being adapted to transmit electromagnetic waves having frequencies exceeding one kilomegacycle per second;

a minor portion of said inner cylinder being hollow and being defined by a thin annular wall;

the remainder of the inner cylinder being solid;

a ceramic wafer hermetically sealed to the outer conductor and to the minor portion of the inner conductor;

said thin annular wall comprising means for relieving thermal stresses at the junction of the ceramic wafer with the inner conductor;

the thickness of the wafer being approximately .030

inch; and

the thickness of the annular wall of the hollow portion of the inner conductor being approximately .008 inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,029,420 Green Feb. 4, 1936 2,529,436 Weber et al. Nov. 7, 1950 2,551,611 Kuhner May 8, 1951 

1. IN COMBINATION: A COAXIAL CABLE HAVING A METAL OUTER CYLINDER SURROUNDING A METAL INNER CYLINDER; A COAXIAL CABLE SECTION HAVING A CERAMIC OUTER CYLINDER SURROUNDING A CERAMIC INNER CYLINDER; SAID SECTION BEING FITTED HERMETICALLY TO SAID COAXIAL CABLE; A FIRST CONDUCTIVE COATING ON THE INNER SURFACE OF THE CERAMIC OUTER CYLINDER; A SECOND CONDUCTIVE COATING ON THE OUTER SURFACE OF THE CERAMIC INNER CYLINDER; AND A CERAMIC WAFER HERMETICALLY SEALED TO THE INNER AND OUTER CERAMIC CYLINDERS OF THE COAXIAL CABLE SECTION. 